Surface modification of carbon fibers

ABSTRACT

A CONTINUOUS PROCESS IS PROVIDED FOR MODIFYING THE SURFACE CHARACTERISTICS OF A CARBONACEOUS FIBROUS MATERIAL (EITHER AMORPHOUS CARBON OR GRAPHIC CARBON) AND TO THEREBY FACILICATE ENHANCED ADHESION BETWEEN THE FIBROUS MATERIAL AND A MATRIX MATERIAL. THE FIBROUS MATERIAL IS CONTINUOUSLY PASSED THROUGH A HEATING ZONE CONTAING GASEOUS CARBON DIOXIDE UNDER CONDITIONS FOUND SUITABLE FOR BRINGING ABOUT THE DESIRED SURFACE MODIFICATION. COMPOSITE ARTICLES OF ENHANCED INTERLAMINAR SHEAR STRENGTH MAY BE FORMED BY INCORPORATING THE FIBERS MODIFIED IN ACCORDANCE WITH THE PRESENT PROCESS IN A RESINOUS MATRIX MATERIAL.

Mm! 21, 197-3 M. L. DRUM ETAL 3,123,150

SURFACE MODIFICATION OF CARBON FIBERS Filed Aug. 20, 1970 INVENTORSMELVIN L [mum GEORGE R. FERMENT VELLIY'UR N. P RAD United States Patent3,723,150 SURFACE MODIFICATION OF CARBON FIBERS Melvin L. Druin, WestOrange, George R. Ferment, Dover, and Velliyur N. P. Rao, NorthPlainfield, N..I., assignors to Celanese Corporation, New York, NY.Filed Aug. 20, 1970, Ser. No. 65,456 Int. Cl. C08h 17/08, 17/10 US. Cl.106-307 4 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTIONIn the search for high performance materials, considerable interest hasbeen focused upon carbon fibers. The term carbon fibers is used hereinin its generic sense and includes graphite fibers as well as amorphouscarbon fibers. Graphite fibers are defined herein as fibers whichconsist essentially of carbon and have a predominant X-ray difi'ractionpattern characteristic of graphite. Amorphous carbon fibers, on theother hand, are defined as fibers in which the bulk of the fiber weightcan be attributed to carbon and which exhibit an essentially amorphousX-ray diifraction pattern. Graphite fibers generally have a higherYoungs modulus than do amorphous carbon fibers and in addition are morehighly electrically and thermally conductive.

Industrial high performance materials of the future are projected tomake substantial utilization of fiber reinforced composites, and carbonfibers theoretically have among the best properties of any fiber for useas high strength reinforcement. Among these desirable properties arecorrosion and high temperature resistance, low density, high tensilestrength, and high modulus. Graphite is one of the very few knownmaterials whose tensile strength increases with temperature. Uses forcarbon fiber reinforced composites include aerospace structuralcomponents, rocket motor casings, deep-submergence vessels and ablativematerials for heat shields on re-entry vehicles.

In the prior art numerous materials have been proposed for use aspossible matrices in which carbon fibers may be incorporated to providereinforcement and produce a composite article. The matrix material whichis selected is commonly a thermosetting resinous material and iscommonly selected because of its ability to also withstand highlyelevated temperatures.

While it has been possible in the past to provide carbon fibers ofhighly desirable strength and modulus characteristics, difficulties havearisen when one attempts to gain the full advantage of such propertiesin the resulting carbon fiber reinforced composite article. Suchinability to capitalize upon the superior single filament properties ofthe reinforcing fiber has been traced to inadequate adhesion between thefiber and the matrix in the resulting composite article.

Various techniques have been proposed in the past for modifying thefiber properties of a previously formed carbon fiber in order to makepossible improved adhesion when present in a composite article. See, forinstance,

British Pat. No. 1,180,441 to Nicholas J. Wadsworth and William Wattwherein it is taught to heat a carbon fiber normally within the range of350 C. to 850 C. (e.g. 500 to 600 C.) in an oxidizing atmosphere such asair for an appreciable period of time. Other atmospheres contemplatedfor use in the process include an oxygen rich atmosphere, pure oxygen,or an atmosphere containing an oxide of ntirogen from which free oxygenbecomes available such as nitrous oxide and nitrogen dioxide.

It is an object of the invention to provide a continuous process forefiiciently modifying the surface characteristics of carbon fibers.

It is an object of the invention to provide a process for improving theability of carbon fibers to bond to a resinous matrix material.

It is an object of the invention to provide a process for modifying thesurface characteristics of carbon fibers which may be conductedrelatively rapidly.

It is another object of the invention to provide composite articlesreinforced with carbon fibers exhibiting improved interlaminar shearstrength.

These and other objects, as well as the scope, nature, and utilizationof the invention will be apparent from the following detaileddescription and appended claims.

SUMMARY OF THE INVENTION It has been found that a process for themodification of the surface characteristics of a carbonaceous fibrousmaterial containing at least about percent carbon by weight comprisescontinuously passing a continuous length of said fibrous materialthrough a heating zone provided at a temperature of about 700 to 1800 C.containing a gaseous atmosphere consisting essentially of about 0.5 to100 percent by volume of carbon dioxide and about 0 to 99.5 percent byvolume of an inert gas for a residence time of about 3 seconds to 1hour.

The resulting carbon fibers may be incorporated in a resinous matrixmaterial to form a composite article exhibiting enhanced interlaminarshear strength.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a photograph made with the aid ofa scanning electron microscope of a portion of graphite filament whichhas not undergone surface modification.

FIG. 2 is a photograph made with the aid of a scanning electronmicroscope of a portion of a graphite filament which has been surfacemodified in accordance with the present process.

FIG. 3 is a photograph made with the aid of a scanning electronmicroscope of a portion of a graphite filament which has undergonesurface modification while employing other than optimum surfacetreatment conditions.

DESCRIPTION OF PREFERRED EMBODIMENTS The starting material The fiberswhich are modified in accordance with the present process arecarbonaceous and contain at least about 90 percent carbon by weight.Such carbon fibers may exhibit either an amorphous carbon or apredominantly graphitic carbon X-ray diffraction pattern. In a preferredembodiment of the process the carbonaceous fibers which undergo surfacetreatment contain at least about percent carbon by weight, and at leastabout 99' percent carbon by weight in a particularly preferred embodiment of the process.

The carbonaceous fibrous materials may be present as a continuous lengthin a variety of physical configurations provided substantial access tothe fiber surface is possible during the surface modification treatmentdescribed hereafter. For instance, the carbonaceous fibrous materialsmay assume the configuration of a continuous length of a multifilamentyarn, tape, tow, strand, cable, or similar fibrous assemblage. In apreferred embodiment of the process the carbonaceous fibrous material isone or more continuous multifilament yarn. When a plurality ofmultifilament yarns are surface treated simultaneously, they may becontinuously passed through the heating zone while in parallel and inthe form of a flat ribbon.

The carbonaceous fibrous material which is treated in the presentprocess optionally may be provided with a twist which tends to improvethe handling characteristics. For instance, a twist of about 0.1 tot.p.i., and preferably about 0.3 to 1.0 t.p.i., may be imparted to amultifilament yarn. Also, a false twist may be used instead of or inaddition to a real twist. Alternatively, one may select continuousbundles of fibrous material which possess essentially no twist.

The carbonaceous fibers which serve as the starting material in thepresent process may be formed in accordance with a variety of techniquesas will be apparent to those skilled in the art. For instance, organicpolymeric fibrous materials which are capable of undergoing thermalstabilization may be initially stabilized by treatment in an appropriateatmosphere at a moderate temperature (e.g. 200 to 400 C.), andsubsequently heated in an inert atmosphere at a more highly elevatedtemperature, e.g. 900 to 1000 C., or more, until a carbonaceous fibrousmaterial is formed. If the thermally stabilized material is heated to amaximum temperature of 2000 to 3100 C. (preferably 2400 to 3100 C.) inan inert atmosphere, substantial amounts of graphitic carbon arecommonly detected in the resulting carbon fiber, otherwise the carbonfiber will commonly exhibit an essentially amorphous X-ray diffractionpattern.

The exact temperature and atmosphere utilized during the initialstabilization of an organic polymeric fibrous material commonly varywith the composition of the precursor as will be apparent to thoseskilled in the art. During the carbonization reaction elements presentin the fibrous material other than carbon (e.g. oxygen and hydrogen) aresubstantially expelled. Suitable organic polymeric fibrous materialsfrom which the fibrous material capable of undergoing carbonization maybe derived include an acrylic polymer, a cellulosic polymer, apolyamide, a polybenzimidazole, polyvinyl alcohol, etc. As discussedhereafter, acrylic polymeric materials are particularly suited for useas precursors in the formation of carbonaceous fibrous materials.Illustrative examples of suitable cellulosic materials include thenatural and regenerated forms of cellulose, e.g. rayon. Illustrativeexamples of suitable polyamide materials include the aromaticpolyamides, such as nylon 6T, which is formed by the condensation ofhexamethylenediamine and terephthalic acid. An illustrative example of asuitable polybenzimidazole ispoly-2,2'-m-phenylene-5,5'-bibenzimidazole.

A fibrous acrylic polymeric material prior to stabilization may beformed primarily of recurring acrylonitrile units. For instance, theacrylic polymer should contain not less than about 85 mole percent ofrecurring acrylonitrile units with not more than about 15 mole percentof a monovinyl compound which is copolymerizable with acrylonitrile suchas styrene, methyl acrylate, methyl methacrylate, vinyl acetate, vinylchloride, vinylidene chloride, vinyl pyridine, and the like, or aplurality of such monovinyl compounds.

During the formation of a preferred carbonaceous fibrous material foruse in the present process multifilament bundles of an acrylic fibrousmaterial may be initially stabilized in an oxygen-containing atmosphere(i.e. preoxidized) on a continuous basis in accordance with theteachings of US. Ser. No. 749,957, filed Aug. 5, 1968, of Dagobert E.Stuetz, which is assigned to the same assignee as the present inventionand is herein incorporated by reference. More specifically, the acrylicfibrous material should be either an acrylonitrile homopolymer or anacrylonitrile copolymer which contains no more than about 5 mole percentof one or more monovinyl comonomers copolymerized with acrylonitrile. Ina particularly preferred embodiment of the process the fibrous materialis derived from an acrylonitrile homopolymer. The stabilized acrylicfibrous material which is preoxidized in an oxygen-containing atmosphereis black in appearance, contains a bound oxygen content of at leastabout 7 percent by weight as determined by the Unterzaucher analysis,retains its original fibrous configuration essentially intact, and isnon-burning when subjected to an ordinary match flame.

In preferred techniques for forming the starting material for thepresent process a stabilized acrylic fibrous ma terial is carbonized andgraphitized while passing through a temperature gradient present in aheating zone in accordance with the procedures described in commonlyassigned U.S. Ser. Nos. 777,275, filed Nov. 20, 1968 of Charles M.Clarke; 17,780, filed Mar. 9, 1970 of Charles M. Clarke, Michael 1. Ram,and John P. Riggs; and 17,832, filed Mar. 9, 1970 of Charles M. Clarke,Michael J. Ram, and Arnold J. Rosenthal. Each of these disclosures isherein incorporated by reference.

In accordance with a particularly preferred carbonization andgraphitization technique a continuous length of stabilized acrylicfibrous material which is non-burning when subjected to an ordinarymatch flame and derived from an acrylic fibrous material selected fromthe group consisting of an acrylonitrile homopolymer and acrylonitrilecopolymers which contain at least about mole percent of acrylonitrileunits and up to about 15 mole percent of one or more monovinyl unitscopolymerized therewith is converted to a graphitic fibrous materialwhile preserving the original fibrous configuration essentially intactwhile passing through a carbonization/graphitization heating zonecontaining an inert gaseous atmosphere and a temperature gradient inwhich the fibrous material is raised within a period of about 20 toabout 300 seconds from about 800 C. to a temperature of about 1600 C. toform a continuous length of carbonized fibrous material, and in whichthe carbonized fibrous material is subsequently raised from about 1600"C. to a maximum temperature of at least about 2400 C. within a period ofabout 3 to 300 seconds Where it is maintained for about 10 seconds toabout 200 seconds to form a continuous length of graphitic fibrousmaterial.

The equipment utilized to produce the heating zone used to produce thecarbonaceous starting material may be varied as will be apparent tothose skilled in the art. It is essential that the apparatus selected becapable of producing the required temperature While excluding thepresence of an oxidizing atmosphere.

In a preferred technique the continuous length of fibrous materialundergoing carbonization is heated by use of an induction furnace. Insuch a procedure the fibrous material may be passed in the direction ofits length through a hollow graphite tube or other susceptor which issituated within the windings of an induction coil. By varying the lengthof the graphite tube, the length of the induction coil, and the rate atwhich the fibrous material is passed through the graphite tube, manyapparatus arrangements capable of producing carbonization orcarbonization and graphitization may be selected. For large scaleproduction, it is of course preferred that relatively long tubes orsusceptors be used so that the fibrous material may be passed throughthe same at a more rapid rate while being carbonized or carbonized andgraphitized. The temperature gradient of a given apparatus may bedetermined by conventional optical pyrometer measurements as will beapparent to those skilled in the art. The fibrous material because ofits small mass and relatively large surface area instantaneously assumesessentially the same temperature as that of the zone through which it iscontinuously passed.

The surface treatment The continuous length of carbonaceous fibrousmaterial is continuously passed (e.g. in the direction of its length)through a heating zone containing a gaseous atmosphere consistingessentially of about 0.5 to 100' percent by volume of carbon dioxide(preferably 5 to 100 percent by volume carbon dioxide) and about 0 to99.5 percent by volume of an inert carrier gas (preferably 0 to 95percent by volume inert carrier gas) under the conditions described indetail hereafter. Suitable inert carrier gases include nitrogen, argon,and helium, etc. In a particularly preferred embodiment of the processthe gaseous atmosphere of the heating zone is essentially pure carbondioxide thereby eliminating the need to supply more than one gas to theheating zone as well as the difficulties connected with the feeding of aplurality of gases to produce a gaseous mixture of the desiredconcentration. It is recommended that molecular oxygen be excluded fromthe heating zone, however, trace amounts of molecular oxygen (e.g. up toabout 2 percent by volume) can generally be tolerated in combinationwith the active carbon dioxide species without deleterious results.

The gaseous atmosphere (heretofore described) is provided in the heatingzone at a temperature of about 700 to 1800 C. At temperatures much belowabout 700 C. the surface treatment reaction tends to be inordinatelyslow. At temperatures much above about 1800 C. the surface treatmentreaction becomes so rapid that it is difiicult to control. If desired atemperature gradient may be provided within the heating zone which risesto the desired surface treatment temperature. The gaseous atmospherepreferably is preheated prior to introduction into the heating zone andpreferably is continuously supplied to the heating zone with a portionof the gaseous atmosphere being continuously withdrawn from the heatingzone whereby olf gases are effectively expelled. In a preferredembodiment of the process wherein the gaseous atmosphere is essentiallypure carbon dioxide the gaseous atmosphere is provided at a temperatureof about 900 to 1300 C.

The contact time during which the carbonaceous fibrous material ispassed through the heating zone commonly ranges from about 3 seconds to1 hour. The minimum contact time varies with the concentration of carbondioxide in the gaseous atmosphere, the temperature of the gaseousatmosphere, and the relative molar concentrations of carbon dioxide andcarbon present in the carbonaceous fibrous material within the heatingzone. Generally the higher the temperature of the carbon dioxidecontaining gaseous atmosphere, the more rapid the surface modification.Generally the higher the concentration of carbon dioxide in the gaseousatmosphere, the more rapid the surface modification. Also it has beenobserved that graphitic fibrous materials of high Young's modulus (e.g.in excess of 50,- 000,000 p.s.i.) tend to require a slightly longercontact time for optimum results than do carbonaceous fibrous materialsof a predominantly amorphous X-ray diffraction pattern which generallyexhibit a lower Youngs modulus. Also when the carbonaceous fibrousmaterial is provided as a relatively compact assemblage of a pluralityof fibers, then longer residence times may be advantageously employed aswill be apparent to those skilled in the art.

The surface modification treatment of the present process is generallyterminated prior to achieving a fiber weight loss much in excess ofpercent by weight. Greater fiber weight losses are to be avoided sincesuch weight losses are generally indicative of an excessive surfacetreatment and yield no commensurate advantage. In fact, theeffectiveness of the surface treatment previously achieved may actuallybe diminished in some instances. Fiber weight losses of about 0.5 to 7percent by weight (e.g. 1 or 2 percent by weight) are commonly attainedin particularly preferred embodiments of the present process.

A particularly preferred embodiment of the present process for themodification of the surface characteristics of a carbonaceous fibrousmaterial containing at least about percent carbon by weight andexhibiting a predominantly graphitic X-ray diffraction patterncomprises: (a) continuously introducing a continuous length of thefibrous material into a heating zone provided at a temperature of about900 to 1300 C. containing a gaseous carbon dioxide atmosphere, (b)continuously introducing essentially pure gaseous carbon dioxide intosaid heating zone, (c) continuously withdrawing a portion of the gaseousatmosphere from said heating zone, (d) continuously passing saidcontinuous length of carbonaceous fibrous material through said heatingzone at said temperature for a residence time of about 3 to 240 seconds,and (e) continuously withdrawing the resulting continuous length ofcarbonaceous fibrous material from said heating zone.

The theory whereby the surface of a carbonaceous fibrous material ismodified in the present process is considered complex and incapable ofsimple explanation. It is believed, however, that the resultingmodification is attributable to a combination of physical and chemicalinteractions between the gaseous atmosphere and the carbonaceous fibrousmaterial. Such interaction likely includes the chemical reaction ofcarbon dioxide with carbon adjacent the surface of the fiber to yieldcarbon monoxide. Such carbon monoxide may be continuously withdrawn fromthe heating zone together with any other off gases which are evolved.

The surface modification imparted to the carbonaceous fibrous materialthrough the use of the present process has been found to exhibit anappreciable life which is not diminished to any substantial degree evenafter the passage of 30, or more days.

The surface treatment of the present process makes possible improvedadhesive bonding between the carbonaceous fibers, and a resinous matrixmaterial. Accordingly, carbon fiber reinforced composite materials whichincorporate fibers treated as heretofore described exhibit enhancedshear strength, fiexural strength, compressive strength, etc. Theresinous matrix material employed in the formation of such compositematerials is commonly a polar thermosetting resin such as an epoxy, apolyimide, a polyester, 2. phenolic, etc. The carbonaceous fibrousmaterial is commonly provided in such resulting composite materials ineither an aligned or random fashion in a concentration of about 20 to 70percent by volume.

The following examples are given as specific illustrations of theinvention. It should be understood, however, that the invention is notlimited to the specific details set forth in the examples.

EXAMPLE I A high strength-high modulus carbonaceous yarn derived from anacrylonitrile homopolymer yarn in accordance with procedures describedin US. Ser. No. 749,957, filed Aug. 5, 1968, and 777,275 filed Nov. 20,1968 was selected as the starting material. The yarn consisted of a 1600fil. bundle having a total denier of about 1000, had a carbon content inexcess of 99 percent by weight, exhibited a predominantly graphiticX-ray diffraction pattern, a single filament tenacity of about 358,000p.s.i., and a single filament Youngs modulus of about 115,000,- 000p.s.i. A photograph of a filament of the untreated yarn made with theaid of a scanning electron microscope at a magnification of 6400 isprovided as FIG. 1.

Portions of the yarn were continuously unwound from bobbins and 15 endsof the yarn were continuously passed while in parallel and in the formof a fiat ribbon at various rates through a heat treatment zone providedwith a temperature gradient containing an atmosphere of essentially purecarbon dioxide.

The heat treatment zone consisted of an 18 inch Inconel tube having aninner diameter of about 1 inch which was positioned within a resistancewound mufiie furnace having a length of 12 inches. Three inches of theInconel tube protruded from each end of the muffle furnace. -A hot zone(maximum temperature portion of gradient) having a length of about 3inches was centrally located in the Inconel tube through which the yarncontinuously passed and was adjusted to a constant temperature of about1050 C.

Gaseous carbon dioxide was continuously introduced into the Inconel tubeat the yarn feed end at a rate of 25 s.c.f.h. (std. cu. ft. per hour).Air was excluded from the heat treatment zone by means of a nitrogenpadded chamber which enclosed the surface treatment chamber. Off gaseswere continuously displaced and withdrawn from the heat treatment zoneby the continuously introduced gas supply. Off gases were withdrawn fromthe surface treatment zone primarily at the yarn exit end of the tube.The fiber weight losses which occurred during the surface treatment wereless than 10 percent, e.g. commonly 1 to 3 percent.

Composite articles were next formed employing the surface modified yarnsamples as a reinforcing medium in a resinous matrix. The compositearticles were rectangular bars consisting of about 50 percent by volumeof the yarn and having dimensions of inch x 4 inch x 5 inches. Thecomposite articles were formed by impregnation of the yarn in a liquidepoxy resin-hardener mixture at 50 C. followed by unidirectional layupof the required quantity of the impregnated yarn in a steel mold andcompression molding of the layup for 2 hours at 93 C., and 2.5 hours at200 C. in a heated platen press at about 100 p.s.i. pressure. The moldwas cooled slowly to room temperature, and the composite article wasremoved from the mold cavity and cut to size for testing. The resinousmatrix material used in the formation of the composite article wasprovided as a solventless system which contained 100 parts by weight ofepoxy resin and 88 parts by weight of anhydride curing agent.

The following data summarizes the surface treatment conditions employedand the properties achieved.

Singlefilament Interlaminar Yarn Time at tenacity after shear strengthThe horizontal interlaminar shear strengths reported were determined byshort beam testing of the carbon fiber reinforced composite according tothe procedure of ASTM D2344-65T as modified for straight bar testing ata 4:1 span to depth ratio.

For comparative purposes a composite article was formed as heretoforedescribed employing an identical carbonaceous yarn without subjectingthe same to any form of surface modification. The average horizontalinterlaminar shear strength of the composite article was only 3000p.s.i.

A photograph of a filament of the surface treated yarn of Sample C(above) made with the aid of a scanning electron microscope at amagnification of 6400 is pro vided as FIG. 2.

EXAMPLE H Example I was repeated with the exception that the three inchhot zone of the Inconel tube was provided at a temperature of 1135 C.

The following data summarizes the surface treatment conditions employedand the properties achieved.

Singlefilament Interlaminar Yarn Time at tenacity after shear strengthspeed, 1,050 C. surface treatof composite,

Sample in./min. in seconds ment, p.s.i. p.s.i.

EXAMPLE III Example I was repeated with the exception that the threeinch hot zone of the Inconel tube was provided at a temperature of 1250C.

The following data summarizes the surface treatment conditions employedand the properties achieved.

Singlefilament Interlaminar Yarn Time at tenacity after shear strengthspeed, 1,050 C. surface treatol composite,

Sample lnJmin. in seconds ment, p.s.i. p.s.i.

A comparison of the properties achieved indicates that EXAMPLE IV A highstrength-high modulus yarn substantially similar to that employed inExamples I-III was selected as the starting material. The yarn consistedof a 1600 fil bundle having a total denier of about 1000, had a carboncontent in excess of 99 percent by weight, exhibited a predominantlygraphic X-ray diffraction pattern, a single filament tenacity of 302,000p.s.i., and a single filament Youngs modulus of about 87,000,000 p.s.i.

Portions of the yarn were surface treated and formed into composites asdescribed in Example I.

The following data summarizes the surface treatment conditions employedand the properties achieved when the hot zone of the Inconel tube wasprovided at 900 C.

Single filament Interlamlnar Yarn Time at tenacity after shear strengthspeed, 900 surface treatof composite, Sample in./min. in seconds ment,p.s.i. p.s.i.

EXAMPLE V A high strength-high modulus yarn substantially similar tothat employed in Examples I-IV was selected as the starting material.The yarn consisted of a 1600 fil bundle having a total denier of about1000, had a carbon content in excess of 99 percent by weight, exhibiteda predominantly graphitic X-ray diffraction pattern, a single filamenttenacity of about 281,000 p.s.i., and a single filament Youngs modulusof 87,000,000 p.s.i.

Portions of the yarn were surface treated and formed into composites asdescribed in Example I.

The following data summarizes the surface treatment conditions employedand the properties achieved when the hot zone of the Inconel tube wasprovided at 1050 C.

Single filament Interlaminar Yarn Time at tenacity after shear strengthspeed, 1,050 0. surface treatof composite,

Sample in./nn'n. in seconds ment, p.s.i. p.s.i.

A comparison of the properties achieved indicates that Sample B receivedmore than the optimum degree of surface treatment. Additionally, a fiberweight loss of 22 percent was experienced in Sample B, while a weightloss of only 7.5 percent was experienced in Sample A.

For comparative purposes the surface treatment of a 10 which waspositioned within a graphite susceptor of an indication furnace having alength of 42 inches.

The premixed gaseous atmosphere was continuously introduced into theyarn feed end of the ceramic tube at a rate of 25.0 s.c.f.h. Air wasexcluded from the heat treatment zone by means of a nitrogen paddedchamber which enclosed the heat treatment furnace. Off gases werecontinuously displaced and withdrawn from the heat treatment zone by thecontinuously introduced gas supply. Ofi? gases were withdrawn from thesurface treatment zone primarily at the yarn exit end of the tube.

Portions of the yarn were surface treated and formed into composites aspreviously described.

The following data summarizes the surface treatment conditions employedand the properties achieved when the hot zone of the ceramic tube wasprovided at 1700 C.

carbonaceous fiber was attempted employing essentially pure carbonmonoxide in the heating zone. The bonding characteristics of the fiberto a matrix material were not enhanced.

EXAMPLE VI A high strength-high modulus yarn substantially similar tothat employed in Examples IV was selected as the starting material. Theyarn consisted of a 1600 fil bundle having a total denier of about 1000,had a carbon content in excess of 99 percent by weight, exhibited apredominantly graphitic X-ray diffraction pattern, a single filamenttenacity of about 340,000 p.s.i., and a single filament Youngs modulusof about 90,000,000 p.s.1.

Portions of the yarn were surface treated and formed into composites asdescribed in Example I with the exception that a premixed gaseousmixture of carbon dioxide and nitrogen was utilized in the heating zone.

The following data summarizes the surface treatment conditions employedand the properties achieved when Single Volume filament InterlaminarTime at percent tenacity after shear strength 1700 0. 002111 surfacetreatof composite, seconds mixture ment, p.s.i. p.s.i.

The resulting surface modified carbonaceous fiber may next be utilizedas a reinforcing medium in the formation composite articles byincorporation in a resinous matrix material.

Although the invention has been described with preferred embodiments, itis to be understood that variations and modifications may be resorted toas will be apparent to those skilled in the art. Such variations are tobe considered within the purview and scope of the claims appendedhereto.

We claim:

1. An improved process for the modification of the surfacecharacteristics of a carbonaceous fibrous material containing at leastabout 95 percent carbon by weight and exhibiting a predominantlygraphitic X-ray diffraction pattern so as to improve its ability to bondto a resinous matrix material comprising:

(a) continuously introducing a continuous length of said carbonaceousfibrous material into a heating zone provided at a temperature of about900 to 1300 C. containing a gaseous carbon dioxide atthe hot zone of theInconel tube was provided at 1280 C. mosphere,

Single Volume filament Interlarninar Yarn Time at percent tenacity aftershear strength speed, 1,280 C. CO; in surface treatof composite, Samplelmln. seconds mixture merit, p.s.i. p.s.i.

A 10 15 7. 4 282, 000 7, 065 B 10 15 16. o 272, 000 10, 345 C 10 16 33.0 300, 000 10, 290

[EXAMPLE VII Example VI was repeated employing a substantially similarhigh strength-high modulus yarn, and a different apparatus capable ofproducing a temperature gradient having a 10 inch hot zone at atemperature of about 1700 C.

The yarn consisted of a 1600 fil bundle having a total denier of about1000, had a carbon content in excess of 99 percent by weight, exhibiteda predominantly graphitic X-ray diffraction pattern, a single filamenttenacity of about 326,000 p.s.i., and a single filament Youngs modulusof about 90,000,000 p.s.i.

The heat treatment zone consisted of a 48 inch long ceramic tube havingan inner diameter of about 0.5 inch 2. An improved process according toclaim 1 wherein said carbonaceous fibrous material contains at leastabout 99 percent carbon by weight.

3. An improved process according to claim 1 wherein said carbonaceousfibrous material is derived from an acrylic fibrous material selectedfrom the group consisting of an acrylonitrile homopolymer andacrylonitrile copolymers which contain at least about 85 mole percent ofacrylonitrile units and up to about 15 mole percent of one or moremonovinyl units copolymerized therewith.

4. An improved process according to claim 1 wherein said continuouslength of carbonaceous fibrous material 15 is one or more continuousmultifilament yarn.

1 2 References Cited UNITED STATES PATENTS 3,476,703 11/ 1969 Wadsworth106-307 OTHER REFERENCES Modern Refractory Practice, Harbison-WalkerRefractories Co., Pittsburgh, Pa., 1961, page 42.

Such et al.: Process and Apparatus for Treatment of Carbon or GraphiteFibers, Chem. Abstracts, vol. 71, 1969 (col. 103026h).

JAMES E. POER, Primary Examiner US. Cl. X.R. 23209.1, 209.2

